1. Field of the Invention
The present invention relates to a hitless path switching apparatus and method in digital communication systems, and particularly to a hitless path switching apparatus and method suitable for SDH (Synchronous Digital Hierarchy), SONET (Synchronous Optical Network), and ATM (Asynchronous Transfer Mode) transmission systems.
2. Description of Related Art
A transmission system normally employs a redundant system which consists of one or more working systems and one protection system to establish highly reliable communications.
FIG. 1 is a block diagram showing a conventional redundant system. In this figure, a working path 3 and a protection path 4, each including a plurality of regenerative repeaters, are installed between two line terminals 1 and 2. If some failure occurs in the working path 3, the communications between the line terminals 1 and 2 can be continued by switching from the working path 3 to the protection path 4.
Switching from the working path to the protection path in such a conventional redundant system usually involves a service interruption. More specifically, actual switching processing requires a series of processings such as notification of the occurrence of a failure from the receiving end to the transmitting end, confirmation of a normally operable state of the protection system, a switching operation, and reframing of line signals through the protection path. Thus, it is inevitable that a instantaneous service interruption occurs in such redundant systems. Since the data loss due to the instantaneous interruption increases with the transmission bit rate, this presents a large problem.
FIG. 2 shows a conventional path switching apparatus proposed to solve such a problem. It is disclosed in Japanese Patent Application Laying-open No. 344104/1993 by Uematsu et al., and FIG. 2 shows a receiving side of a transmission system.
Input line signals from a working path 11 and a protection path 21 are supplied to interface circuits 13 and 23 through input ports 12 and 22, respectively. The interface circuits 13 and 23 carry out optical-to-electrical conversion and regeneration of the received line signals, and supply their outputs to signal terminating circuits 14 and 24, respectively. The signal terminating circuits 14 and 24 perform line signal termination such as frame alignment and bit error detection by using parity checking, and supply their outputs to delay circuits 15 and 25. The delay circuits 15 and 25 provide the signals with a delay time longer than that required for frame-phase matching of the two line signals. Signal-failure detecting circuits 16 and 26 are connected to the interface circuits 13 and 23, respectively, and provide a switching circuit 30 with switching control signals upon detecting an input line signal failure. The signal terminating circuits 14 and 24 also provide the switching circuit 30 with switching control signals upon detecting failure in the working path. The switching circuit 30 switches to the protection path if a failure occurs in the working path.
The path switching apparatus as shown in FIG. 2 detects a line signal failure by the signal-failure detecting circuits 16 and 26, and a bit-error by the signal terminating circuits 14 and 24. Generally speaking, it takes a considerable time to determine the occurrence of an unexpected failure and to generate the corresponding alarm because of a protection time assigned to determine a loss of frames, a loss of an optical input signal or signal degradation of an input signal. Bit errors are usually detected using a bit interleaved parity code check on a super frame, and the signal degradation is determined if the bit errors breaking a threshold are detected on some sequent super frames, the number of which is defined as the protection time. For example, if we are to detect the bit error of 10.sup.-6 on a data block, the length of a super frame should be more than 10.sup.6 bits, which corresponds to about 6.4 ms if the transmission bit rate is set at 155.52 Mbit/s. So the signal degradation detection also takes a considerable time.
Accordingly, a switching operation from the working path to the protection path after determining the occurance of a failure cannot prevent information data including a number of lost bits from being sent to the downstream apparatus.